KVM: x86/mmu: Try "unprotect for retry" iff there are indirect SPs

Try to unprotect shadow pages if and only if indirect_shadow_pages is non-
zero, i.e. iff there is at least one protected such shadow page.  Pre-
checking indirect_shadow_pages avoids taking mmu_lock for write when the
gfn is write-protected by a third party, i.e. not for KVM shadow paging,
and in the *extremely* unlikely case that a different task has already
unprotected the last shadow page.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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